Oxygen potentials and phase equilibria in the system Ca–Co–O and thermodynamic properties of Ca3Co2O6 and Ca3Co4O9.163

•Improved definition of cation and oxygen nonstoichiometry of Ca3Co3.93+αO9.36−δ.•Measurement of ΔμO2ΔμO2 associated with two 3-phase fields as a function of temperature.•Use of solid-state electrochemical cells for accurate measurement of ΔμO2ΔμO2.•Decomposition temperatures and thermodynamic properties for ternary oxides.•Characterization of ternary phase diagram of the system Ca–Co–O.

Oxygen potentials established by the equilibrium between three condensed phases, CaOss+CoOss+Ca3Co2O6 and CoOss+Ca3Co2O6+Ca3Co3.93+αO9.36−δ, are measured as a function of temperature using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen as the reference electrode. Cation non-stoichiometry and oxygen non-stoichiometry in Ca3Co3.93+αO9.36−δ are determined using different techniques under defined conditions. Decomposition temperatures and thermodynamic properties of Ca3Co2O6 and Ca3Co4O9.163 are calculated from the results. The standard entropy and enthalpy of formation of Ca3Co2O6 at 298.15 K are evaluated. Using thermodynamic data from this study and auxiliary information from the literature, phase diagram for the ternary system Ca–Co–O is computed. Isothermal sections at representative temperatures are displayed to demonstrate the evolution of phase relations with temperature.

Graphical abstractIsothermal section of the phase diagram of the system Ca–Co–O at 1250 K.Figure optionsDownload full-size imageDownload as PowerPoint slide